Semiconductor manufacturing has historically used 18 MOhm/cm deionized water as a rinsing and cleaning fluid. Semiconductor manufacturers put water through a sequence of steps to remove particles and metal ions that would otherwise poison semiconductor devices. Once these impurities are removed from water the resistivity is typically raised to 18 MOhm/cm. DI water in many cases is used as a pressurized medium to remove particles from the surfaces of wafers. These particles would otherwise cause defects in the semiconductor devices on the wafers. The undesired side effect of pressurized dispense of deionized water on wafers is the build of static voltages and eventually electrostatic discharge (ESD). Many semiconductor devices are ESD sensitive and if voltages are permitted to build and then discharge, yield loss will occur.
The static voltages are created when a non-conductive fluid (such as 18 MOhm/cm DI water) aggressively contacts a non-conductive (or electrically isolated) surface (such as a wafer). The voltage built will be proportional to the aggressiveness (force) of the dispense and proportional to the resistivity of the fluid employed. The cleaning efficiency of the water dispense is also proportional to the force of the spray. This leads to the paradox of high pressure spray being required to remove particles from the wafer surface to increase yield, but high pressure spray causing static voltage to build to the point of discharge causing yield loss.
The industry initially worked around this issue by mixing in chemical agents, such as ammonium hydroxide, to increase the conductivity of the fluid. This was successful but not an environmentally friendly choice, required wastewater streams to be treated and was not an option for many structures that ammonium hydroxide would attack or corrode.
To avoid chemical attack from ammonium hydroxide, carbon dioxide was then employed to lower the resistivity of the DI water. Initial systems had crude controls (such as simply pressurizing tanks with carbon dioxide). These on\off CO2 systems could not accurately maintain the upper resistivity level opening the possibility of ESD damage. Similarly they could not maintain the lower resistivity level which then required excessive quantities of CO2 and opened conditions for carbonic acid to form.
These systems then evolved with more sophisticated controls that permitted holding resistivity levels in the 350 KOhm/cm range within tolerance of 200 KOhm/cm. This was a solution but the semiconductor industry continues to decrease the size of devices, which in turn is making devices more susceptible to ESD damage. The industry now requires lower levels of resistivity and tighter tolerances of control.
The present applicant has responded to industry demands by developing an improved version of its DI water:CO2 mixing system. The new system creates low resistivity (150 KOhm/cm+\−50 KOhm/cm) deionized water on demand.